1. Field of the Invention
This invention relates to a semiconductor laser device, and is particularly effective when applied to a semiconductor laser having a double-hetero structure.
2. Description of the Prior Art
Semiconductor lasers have been put into practical use for optical communication, an optical disk, a laser beam printer, etc., and have become increasingly important from both the aspects of the performance and economy of each system. In order to achieve enhancements in functions, such as realization of a high operating speed, in the future, developments have been put forward for the increase of laser power.
With the present-day GaAlAs/GaAs-based semiconductor laser, a power of about 15 mW is the limit in practical use as regards the room-temperature continuous-wave (cw) oscillation element whose mode is controlled to the fundamental mode. The reason is that when the element is operated at high power, thermal damages occur in the radiation emitting and reflecting planes of a laser resonator (usually, the cleavage planes of a crystal) to degrade the characteristics of the element.
As a measure for improving such laser facet damages, there has been adopted a method in which the element is protected by covering the facets with dielectric films of Al.sub.2 O.sub.3, SiO.sub.2 or the like. As an alternative method, it has been proposed to fabricate the laser in the structure according to which the vicinities of the facets become transparent to laser radiation. An example of this laser is one disclosed in Japanese Patent Application Publication No. 55-27474 wherein a Zn diffusion region reaching an active layer is disposed in only the central part of a resonator without forming any diffusion region near the facets. In this example, the region in which laser emission takes place has, in effect, the band gap (E.sub.g) narrowed as compared with that in the vicinities of the facets because the diffusion of Zn gives rise to emission transition between the conduction band and the acceptor level of Zn. This is equivalent to the form in which the facets are protected by transparent members.
However, the following are mentioned as the difficulties of these structures:
(1) In the fabrication, the diffusion front (diffusion depth, width, etc.) needs to be precisely controlled by the Zn diffusion process.
(2) Since any light waveguiding mechanism for transverse modes is not existent near the facets, an output beam is attended with astigmatism. Accordingly, a correcting optical system needs to be installed for utilizing the output beam.